Airway responsiveness to acetylcholine or capsaicin in immature and mature guinea pigs in vivo

ABSTRACTNon-specific airway hyperresponsiveness is a characteristic feature of bronchial asthma. Airway reactivity to cholinergics may change with age during childhood. The present paper describes tests to see if the airway responsiveness induced by acetylcholine (ACh) or capsaicin, which causes a release of endogenous tachykinins from the sensory nerve, changes with age in guinea pigs. Changes in lung resistance (RL) were measured to monitor airflow obstruction caused by intravenous ACh or capsaicin in anesthetized immature guinea pigs (aged 2 weeks), and to compare with those of adult guinea pigs (aged 11 weeks). It was shown that both ACh and capsaicin induced a rapid and dose-dependent increase in RL both in immature and adult animals. The two age groups had similar responsiveness to ACh or capsaicin in terms of potency or sensitivity. The response induced by capsaicin was not affected by atropine. In addition, capsaicin caused no extravasation of Evans Blue dye as an index of airway plasma exudation in both age groups. The airflow obstruction induced by i.v. capsaicin is mainly due to a cholinergic-unrelated smooth muscle contraction. These results suggest that airway smooth muscle responsiveness to cholinergics or capsaicin is comparable in both immature and adult guinea pigs in vivo